Innerseal facings are often used in packaging of such things as foods and medications to seal and secure the opening of a container before a cap is applied. The innerseal facing provides protection from contaminants that may leak through the cap seal, and provides a tamper evident closure for the container.
Conventional innerseal facing materials are exemplified by the Safegard.TM.-100 and Safegard.TM.-75M products commercially available from Minnesota Mining and Manufacturing Company of St. Paul, Minn. The Safegard.TM.-100 product includes an aluminum foil layer coated with a polymeric material. The Safegard.TM.-75M product includes a polyester layer between the foil layer and the polymeric layer. The typical user will cut an appropriately sized innerseal from the facing material and place it into a bottle cap. Next, the cap may be placed upon a plastic bottle that will force the innerseal into intimate contact with the rim of the bottle. When the capped bottles are passed under an induction coil, the aluminum foil will heat up, and transfer heat to the polymeric material. The polymeric material adheres the innerseal to the rim of the container to seal the container. The end user may remove the innerseal from the container rim to gain access to the contents of the bottle.
One problem with traditional innerseal facings has been the wide variation in sealing force associated with the induction sealing process. The heating duration within the induction field is quite short in order to improve throughput, and random process-related variations in heating duration can greatly alter the ultimate bond strength. The range of induction sealing power settings in which acceptable removal forces can be obtained can be relatively narrow. For example, if the container and innerseal are overheated, the polymeric bonding material may be squeezed from the rim area, which results in the aluminum foil bonding directly to the bottle rim. Direct fusing between the aluminum layer and the bottle rim (known as fuse-bonding) is undesirable, in part because it requires a very high removal force to separate the innerseal facing from the container.
Alternate approaches have been proposed to address these and related problems. For example, U.S Pat. No. 5,004,111 to McCarthy shows an innerseal construction with an integral "top" grip tab. The top tab allows the end user to grasp the tab to remove the innerseal facing from the container rim. The sealing layer of this innerseal separates internally to facilitate removal of the innerseal. U.S. Pat. No. 5,012,946 to McCarthy shows an innerseal construction with an integral "edge" grip tab. U.S. Pat. No. 2,620,939 to Weisgerber shows a multiple layer construction for an innerseal for use with glass containers.